On Wed, Apr 26, 2006 at 02:48:53AM -0400, mark@xxxxxxxxxxxxxx wrote: > You said that DB accesses are random. I'm not so sure. In PostgreSQL, > are not the individual pages often scanned sequentially, especially > because all records are variable length? You don't think PostgreSQL > will regularly read 32 bytes (8 bytes x 4) at a time, in sequence? > Whether for table pages, or index pages - I'm not seeing why the > accesses wouldn't be sequential. You believe PostgreSQL will access > the table pages and index pages randomly on a per-byte basis? What > is the minimum PostgreSQL record size again? Isn't it 32 bytes or > over? :-) Data within a page can absolutely be accessed randomly; it would be horribly inefficient to slog through 8K of data every time you needed to find a single row. The header size of tuples is ~23 bytes, depending on your version of PostgreSQL, and data fields have to start on the proper alignment (generally 4 bytes). So essentially the smallest row you can get is 28 bytes. I know that tuple headers are dealt with as a C structure, but I don't know if that means accessing any of the header costs the same as accessing the whole thing. I don't know if PostgreSQL can access fields within tuples without having to scan through at least the first part of preceeding fields, though I suspect that it can access fixed-width fields that sit before any varlena fields directly (without scanning through the other fields). If we ever got to the point of divorcing the in-memory tuple layout from the table layout it'd be interesting to experiment with having all varlena length info stored immediately after all fixed-width fields; that could potentially make accessing varlena's randomly faster. Note that null fields are indicated as such in the null bitmap, so I'm pretty sure that their in-tuple position doesn't matter much. Of course if you want the definitive answer, Use The Source. -- Jim C. Nasby, Sr. Engineering Consultant jnasby@xxxxxxxxxxxxx Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461